1. Field of the Invention
The present invention relates to a film stretching apparatus and a solution film-forming method that is preferably used for manufacturing polymer films for optical purposes such as liquid crystal devices, organic electro luminescence and polarizing plates.
2. Description of the Related Art
Polymer films for use in optical purposes are generally manufactured by a solution film-forming method including the steps of casting a polymer liquid (dope) on a support from a casting die, peeling the solidified dope (polymer film) from the support, drying the film and winding the solidified film.
In order to improve the flatness, mechanical strength and optical properties of the polymer film, Japanese Patent Laid-Open No. 62-115035 describes a tenter apparatus to stretch the polymer film and regulate the width of the polymer film. Such tenter apparatus is provided in the drying step, so the polymer film is stretched between the peeling step and the drying step.
The tenter apparatus is provided with plural clips to hold the lateral edge portions of the polymer film. Each of the clips, fixed to an endless chain at regular intervals, comprises a U-shaped clip body and a swingable flapper attached to the upper portion of the clip body. The flapper is normally held by its weight at a close position in which the polymer film is held between the lower portion of the clip body and the lower end of the flapper. When the upper portion of the flapper is engaged with a clip opener, the flapper rotates upward so that the flapper is held at an release position to allow the lateral edge portions of the polymer film to enter the clip. Then, the upper portion of the flapper is disengaged from the clip opener so that the flapper rotates by its weight from the release position to the close position. Thereby, the lateral edge portions of the polymer film are held by the clip.
Due to the increase in demand for the optical polymer film and the requirement of reducing the manufacture cost, it is necessary to increase the productivity of the polymer film by the solution film-forming method. In other words, it is required to increase the film formation speed and the width of the polymer film. In addition, the polymer film is required to be thinner for the purpose of increasing the optical property and providing various optical functions. The solution film-forming method using the above conventional tenter apparatus can not provide polymer films to satisfy such requirements. For instance, since thin and wide polymer film tends to be curly in the widthwise direction, the clip would not be able to hold properly the lateral edge portions of the polymer film, and thereby, the lateral edge portion of the polymer film may be folded by the clips. Moreover, increasing the film-forming speed causes higher volatility of the solvent of the polymer solution at the time when the clips start to hold the polymer film, and thus the clips cannot hold the polymer film properly. Especially, since the polymer film before the drying step becomes softer due to high film-forming speed, the clips may break the lateral edge portion of the polymer film.
To solve the problem of improper clipping, the inventor of the present invention describes, in Japanese Patent Laid-Open No. 2002-337224, the film stretching method including the steps of initially holding the lateral edge portions of the polymer film by pins, and then holding the polymer film by the clips. This method, however, needs to provide both the pins and the clips so that the tenter apparatus will be more complicated. Moreover, even the pins may not be able to hold the polymer film properly if the lateral edge portions of the polymer films are largely curled.